In a gas-turbine group having sequential combustion consisting of two combustion chambers, when reduced power is required, it is known to effect the part load of gas turbine groups by means of a fuel control system and/or inlet guided vane. In such a gas-turbine group, the output could certainly be lowered by closing the inlet guide vane of the compressor. In addition, power reduction may be adjusted by controlling fuel mass flow feeding the gas turbine group.
In certain cases, when a reason for causing malfunction of the gas turbine group occurs or when a sudden reduction of load is requested, the gas turbine group must be de-loaded as fast as possible in order to avoid any damage. At the same time, it should be noted to take care of reducing the thermal stresses through hot gas path. Generally, fast de-loading, i.e. de-loading gradient faster than regular load gradient during normal operation of the gas turbine group may induce high pulsation in the gas turbine group that potentially cause damage to the combustion chambers.
A method of operating a gas turbine group having sequential combustion is disclosed in U.S. Pat. No. 5,634,327, wherein action is taken first of all on the inlet temperatures at the two turbines in order to provide a partial-load operation. The subsequent lowering of the load takes place by adjustment of the compressor guide blades, that is, by reducing the mass flow to below 50% of the rated load. During this lowering, the inlet temperature at the first turbine is kept constant, while the other inlet temperature at the second turbine is continuously lowered; the outlet temperature of the second turbine is in contrast kept constant. After the lowering of the load is complete by adjusting the compressor guide blades, the inlet temperatures are reduced directly or in a phase-displaced manner.
A method for regulating a gas-turbine assembly is disclosed in U.S. Pat. No. 5,481,865A, in which the gas turbine assembly essentially consists of a compressor unit , of an HP-combustion chamber , of an HP-turbine, of an LP-combustion chamber, of an LP-turbine and of a generator, the rating of the fuel quantity for the HP-combustion chamber is carried out responsive to a corrected temperature signal which is formed by the value of the temperature at the outlet of the LP-turbine, reduced by the particular temperature rise detectable there. This temperature signal is recorded in that the temperature rise generated by the fuel quantity introduced into the LP-combustion chamber is subtracted from the measured temperature at the outlet of the LP-turbine. The uncorrected temperature signal at the outlet of the LP-turbine is used for rating the fuel quantity for the LP-combustion chamber.
A gas turbine engine control method and apparatus is disclosed in U.S. Pat. No. 5,584,171, which is used for optimizing fuel/air mixture, especially during decreasing load conditions. In a large capacity gas turbine engine system used for generation of electric power, the system is designed to operate at a relatively constant rotational speed of the turbine which drives an electric generator unit. The load of the turbine engine varies throughout the day due to changes in electrical power demands. During normal rated load conditions, the control of the fuel/air mixture is by way of changes in fuel supply, corrected to return the turbine exhaust temperature to a desired rated temperature. During this mode of operation, the compressor inlet guide vanes are maintained in a substantially constant open position. For accommodating decreasing load conditions where the temperature drops below a predetermined temperature level below the rated temperature operation, the system is switched to an inlet guide vane control mode of operation. During the guide vane control mode of operation, the inlet guide vanes are controlled at a more precise temperature correction sensitivity than is the case for normal load operations. This more precise inlet guide vane control during decreasing load conditions assures improved fuel/air mixture during decreasing load and consequent improved efficiency and minimal NOx compound emissions, especially during changing ambient temperature conditions where cold air would otherwise increase the air supply.
Even relevant patent documents are referenced as above, there exists the need for a solution to address, or at least mitigate, the high pulsations accompanying fast de-loading of the gas turbine group.